An Ecosystem Model of Intermediate Complexity to test management options for fisheries: A case study

Reduction of complexity and capacity to evaluate management strategies are important considerationswhen modelling complex ecosystems. Models of Intermediate Complexity for Ecosystem assessments(MICE) achieve this by representing interactions among a restricted number of crucial species in theecosystem considered. MICE applications are question-driven, heavily dependent on available data forconditioning and aimed at addressing tactical issues. Here we present the development of a MICE for thecentral Adriatic Sea. This area is important ecologically and economically because it supports a commer-cial stock of Norway lobster (Nephrops norvegicus), and a nursery ground for European hake (Merlucciusmerluccius), as well as being subjected to high levels of fishing pressure. Here, Italian and Croatian bottomtrawl fisheries mainly target Norway lobster while impacting juvenile hake. Single-species stock assess-ments for the target species indicate fluctuating trends between 2006 and 2013, as do the survey biomasstrends of most of their prey, underlining the need for a multispecies management plan. The diet of the tar-get species were examined and prey items partitioned into four groups. Consumption of prey groups bypredators was modelled using a Holling Type II functional response. A modified biomass dynamic modelwas used to represent the prey dynamics, projecting them into the future for 10 years (2013–2022) andtesting different management strategies, including a discard ban. The most effective management optionwas the protection of larger individuals, which led to a clear improvement in the stock abundance andspawning stock biomass of both hake and Norway lobster without significant adverse effects on theirprey. The discard ban scenario appeared to have knock-on effects on both predators and some of theirprey.